Organic polymer coatings can be produced by evaporating an organic monomer and condensing the monomer onto a chilled substrate. The monomer coating is then cured using an electron beam gun or ultraviolet radiation. Typically, the evaporation of the organic monomer is conducted in a vacuum environment. The monomer is sprayed or atomized into an evaporation chamber and vaporized. Temperatures in the evaporation chamber can reach about 550.degree. F. Processes suitable for use with the present invention are disclosed in U.S. Pat. Nos. 4,842,893, 4,954,371, and 5,097,800, which are incorporated herein by reference.
The vaporized monomer builds gas pressure in the evaporation chamber until sufficient pressure is achieved to expel the vapor through a narrow slit in a nozzle positioned at the end of the chamber. The monomer condenses on a substrate placed adjacent to the nozzle opening. Typically, the substrate is placed against a chill drum to cool the surface of the substrate and promote condensation of the monomer. The coated substrate is then passed through a curing station where an electron beam gun or ultraviolet lamp initiates a polymerization reaction. In order to maintain machine direction coating uniformity, the monomer flow into the atomizer or spraying apparatus is precisely controlled. Additionally, the monomer stream and surrounding hardware must be continuously heated at a temperature sufficient to insure continuous atomization and vaporization of the monomer without significant dripping. The precise control of the atomization and vaporization of the monomer also contributes to machine direction or downweb coating uniformity.
However, the precise controls discussed above do not address transverse or cross-direction coating uniformity. Since the evaporation chamber and surrounding hardware must be continuously heated to high temperatures, thermal expansion will alter the machining tolerance of the narrow slot opening in the nozzle. A change in slot tolerance directly affects the transverse or cross-direction coating uniformity. Lack of cross-direction coating uniformity results in coatings of poor quality.
There is a need in the art for a method and apparatus for evaporation of organic monomer at high temperatures that minimizes the effect of thermal expansion upon cross-direction coating uniformity without significantly increasing the cost or complexity of the method and apparatus.